Congenital nystagmus (CN) is a conjugate, rhythmic eye movement disorder characterized by many different waveforms, ranging from jerk to pendular types. The causes of CN are unknown, and no detailed mechanisms have been proposed to explain the generation of the CN waveform. We have been making a detailed study of the eye movements seen in patients with congenital nystagmus. by studying the eye movement waveform under different visual stimulus conditions, we were able to show that instability in the neural integrator was a mechanism consistent with CN. We developed a hypothetical model of congenital nystagmus, and showed that a computer simulation of this model could account for a wide variety of CN waveforms. The basis of this model is an eye velocity feedback signal. This signal could arise from either a proprioceptive afferent signal or a central, efferent, eye velocity signal. In normal subjects, this feedback would have a negative sign and act to increase the time constant of a leaky neural integrator. In patients with CN, the sign of this pathway would be reversed, the positive velocity feedback making the system unstable. By adjusting parameters in the model it was possible to simulate various types of jerk and pendular CN waveforms.